1. Field of Invention
This invention relates generally to hypodermic syringes, and in particular to a safety hypodermic needle and shielding cap assembly, which in its retracted mode permits the user to inject the needle into a patient, and which after the needle is withdrawn from the patient, in its operative mode then shields the point of the needle to prevent accidental sticks thereby.
2. Status of Prior Art
A hypodermic needle is usable for intravenous, subcutaneous and intramuscular injection of fluids, or the removal of blood (venipuncture), body fluids or abnormal collections thereof, the needle being of hollow construction and having a slanted open point. When the needle is mounted on a syringe, it is adapted to aspirate or inject fluids for diagnostic or therapeutic purposes.
Disposable hypodermic needles are now mass produced at low cost, many billions of such syringes being used every year in the health care field. While the modern hypodermic syringe now includes a fluid chamber molded of synthetic plastic material rather than glass, its basic design remains much as it was in 1853 when invented by Charles Pravaz, a French physician.
In a hypodermic syringe of standard design, a piston is slidable within a cylindrical fluid chamber, the shank of the piston extending beyond the rear end of the chamber and terminating in a handle. The front end of the chamber is provided with a projecting nozzle that is coaxial with an internally-threaded socket adapted to receive a needle-supporting hub. When the hub is screwed into the socket, the nozzle is then projected therein to communicate with the needle.
A hypodermic needle and a syringe attached thereto are distributed in sterile condition within a plastic bubble package to protect them against contamination in storage and shipment. In addition, the needle is enclosed in a removable overcap whose inlet end snaps onto the needle hub. Thus, after the hypodermic needle and syringe are removed from its package, in order to put it to use one must first remove the overcap to expose the needle. After the hypodermic syringe has been injected into a patient and then withdrawn, it is the usual practice before discarding the syringe to place the overcap back on the needle hub so that those thereafter handling discarded syringes for purposes of disposal will not be pricked thereby.
When a sterile hypodermic needle is injected into a patient suffering from hepatitis or other infectious diseases and the needle is then withdrawn from the patient, it may be contaminated with infectious agents. Hence, should the handler inadvertently prick himself with this contaminated needle, the consequences may be serious.
The possibility of accidental contamination by needle puncture of the skin of those individuals in the health care field who employ hypodermic syringes for venipuncture, the withdrawal of body fluids or for any other medical purpose is fairly high and represents a significant risk. Thus, physicians, nurses, laboratory personnel, paramedics and others involved in the care and treatment of patients are in danger of being accidentally inoculated with infectious microorganisms by contaminated needles.
Most accidental needle sticks occur when the needles are being recapped; for to do so properly, one must first align the needle with the relatively small diameter inlet of the overcap. Should the needle be misaligned, as may well happen should the handler be careless, distracted or fatigued, the point of the needle will not enter the overcap but may instead puncture the finger of the handler.
It is well established that in the last 25 year the risks involved in handling hypodermic syringes has markedly increased. Statistics indicate very high rates of hepatitis B infection among medical and laboratory personnel by reason of this accidental mode of disease transmission. Medical personnel who care for patients suffering from AIDS run a still higher risk; for a needle contaminated with HIV (Human immunodeficiency virus) is a source of great danger. Should the handler of this needle be accidentally punctured, he faces the prospect of contracting a disease currently having a 100% mortality rate as compared, say, to the 5 to 10% mortality rate of hepatitis B.
The 1988 patent to Wanderer et al., U.S. Pat. No. 4,731,059, is concerned with preventing needle sticks, and for this purpose provides a shield which is slidable from a position covering the needle to a position overlying the fluid chamber so that the needle can be exposed when put to use and thereafter shielded. A somewhat similar arrangement is disclosed in the 1987 patent to Fox, U.S. Pat. No. 4,695,274, which shows a retractable safety jacket for a hypodermic needle.
One practical problem with the safety shields or needle guards of the type disclosed in the Wanderer et al. and Fox patents is that when the needle is exposed so that the syringe can be put to use, the retracted guard then covers and obscures the transparent chamber which is graduated so that one can determine the amount of fluid that is contained therein. Hence this guard interferes with the proper operation of the hypodermic syringe.
But the more serious drawbacks of these prior art needle guard arrangements is that in order to accommodate the guard, they require a modification of the basic configuration of the standard hypodermic syringe. Thus in one commercially available form of safety hypodermic syringe having a needle guard, the needle is mounted on an elongated extension tube projecting from the fluid chamber, so that when the guard is retracted, it overlies the extension tube, not the fluid chamber. Hence, fluid from the chamber must be conducted through the extension tube which, by its very nature, elongates the hypodermic syringe, making it more difficult to handle.
To provide a safety hypodermic needle and shield assembly that is compatible with a standard syringe, the patent to Spier et al., U.S. Pat. No. 4,921,490, and the patent to Sturman et al., U.S. Pat. No. 4,863,435, disclose an assembly having a needle-supporting hub whose hollow base is coupled to the socket of the syringe so that the projecting nozzle then communicates with the needle.
Anchored on this hub and surrounding the needle extending therefrom is a helical spring at whose upper end is a shield, the length of the spring being such as to place the shield in front of the needle point. In the retracted mode of the assembly, the spring is compressed and latched to expose the needle so that it can be injected into a patient. In the operative mode of the assembly, which takes effect after the needle is withdrawn from the patient and the spring is released, the shield then is placed in front of the needle point to prevent accidental sticks.
The safety needle and shield assemblies disclosed in the Spier et al. and Sturman et al. patents have distinct advantages, for they require no modification of the standard syringe. But they have practical drawbacks. One such drawback is that the latching mechanism which releases the spring is so bulky that it prevents injection of the hypodermic needle into a patient at a low angle, as is sometimes necessary. Another drawback is that when the assembly is in its operative position, it may still be possible to elongate the spring and cause the needle to pass through the protective shield and prick the handler.